Planetary gear mechanisms are used for automotive transmissions, and needle roller and cage assemblies are used as bearings for supporting pinions of the planetary gear mechanisms. Referring to FIGS. 5A and 5B, the planetary gear mechanism includes a ring gear (internal gear) 2, a sun gear 4, a plurality of pinions (planetary gears) 6, and a carrier 8. The sun gear 4 is positioned at the center of the large ring gear 2, and the plurality of pinions 6 are interposed between the ring gear 2 and the sun gear 4. The carrier 8 includes pinion shafts 8a, and each pinion shaft 8a supports the pinion 6. Therefore, the pinions 6 can revolve about the sun gear 4 while rotating on their axes.
As illustrated in FIG. 5B, each of the pinions 6 is supported on the pinion shaft 8a in a freely rotatable manner through intermediation of a needle roller and cage assembly 10. The needle roller and cage assembly 10 includes a large number of needle rollers 12 and a cage 14. The needle roller and cage assembly 10 utilizes an outer circumferential surface of the pinion shaft 8a as an inner raceway surface and an inner circumferential surface of the pinion 6 as an outer raceway surface without using a bearing inner race or a bearing outer race. The pinion shaft 8a has an oil lubrication hole 8b formed therein, for supplying lubricating oil. The planetary gear mechanism often has structure of lubricating in the needle roller and cage assembly 10 by drawing-in the lubricating oil through the oil lubrication hole 8b formed in the pinion shaft 8a as described above so as to be guided to the outer circumferential surface of the pinion shaft 8a. 
The pinion 6 is relatively easily distorted due to the action of a centrifugal force or a force applied from the sun gear 4 to the pinion shaft 8a. Due to such a condition, in a case of single-row roller, when the pinion shaft 8a is distorted, an edge stress is liable to be generated in an edge portion of the needle roller 12. In addition, in the planetary gear mechanism for a transmission, a helical gear is used. The helical gear has a torsion angle, and hence a thrust is generated. The thrust acts on the bearing as a moment. With this, the gear is inclined, and hence the contact pressure of the roller edge portion is significantly increased, which is also a factor of the generation of the edge stress. In any case, the edge stress shortens the life of the bearing.
Patent Literature 1 discloses a technology of extending the life of the bearing by arranging the needle rollers in three rows or more and causing differences in roller length and crowning amount of the needle roller for each row. For example, the differences are set to such an extent that the roller length is shortened in an outer row than in an inner row (paragraph 0014 and FIG. 4 of Patent Literature 1), and the crowning amount is increased in the outer row that in the inner row (paragraph 0016 and FIGS. 6 and 7 of Patent Literature 1). Patent Literature 2 discloses a planetary gear-type continuously variable transmission in which the planetary gear is supported by two needle bearings, in other words, a plurality of roller rows are provided (paragraphs 0028 and 0029 and FIG. 1 of Patent Literature 2).
The cage of the needle roller and cage assembly includes pockets for receiving the needle rollers, and in general, a relief portion is formed in each of four corners of the pocket mainly in order to relieve stress concentration. As disclosed in Patent Literature 3, the relief portion is normally formed through a drilling process or a milling process, and hence the relief portion often has an arc shape.